Methods for making foamed articles such as porous polymeric thermoplastic films are well known and include the use of chemical and physical blowing agents. The chemical blowing agents typically decompose at a critical temperature and release a gas within the polymeric films. Physical blowing agents typically are dissolved within the polymeric material and then precipitated to form a foamed structure. The use of such blowing agents has been increasingly discouraged due to potential environmental hazards.
Foaming semi-crystalline polymers using a mixture of thermoplastic polymers and a blowing agent is described for example in U.S. Pat. No. 6,790,870 (DeSimone et al.). Carbon dioxide in liquid, gaseous, or supercritical phase is used as a suitable blowing agent.
Foaming of poly(methyl methacrylate) films using supercritical carbon dioxide (scCO2) is also described by Siripurapu et al. in Macromolecules, 2004 37, 9872-9879 to provide articles with uniform porosity but “thin unfoamed skins” (Conclusion, page 9878).
U.S. Pat. No. 8,828,302B2 (Khine et al.) describes the preparation of thin thermoplastic films having wrinkled surfaces using various methods including treatment with an oxygen plasma while opposite surfaces of the film are constrained. The surface wrinkles can be on the nano- or micrometer scale and can be used as supporting material for cell growth in biological applications.
Trindade et al. in J. of Supercritical Fluids 2017, 217, 125-131 describe the preparation of Janus spheres having wrinkled surfaces of periodic patterns using a multi-step process of UV irradiation and swelling with scCO2. It was found, however, that the generated wrinkles could be erased with time and temperature. The authors also allege that the specific polymer composition affects the type of wrinkled surface (Conclusions, page 130).
It is desirable to find a way to create a porous organic polymeric film that also has a wrinkled or textured surface. It is this problem that is solved by the present invention.